1. Field of the Invention
This invention relates to apparatus and methods for controlling the contrast of an image on a pixel by pixel basis. More specifically, this invention relates to apparatus and methods for utilizing a liquid-crystal-on-silicon optically addressed pixel array, with on-pixel circuitry to attenuate the optical output in proportion to the intensity of the input light for intensities that are above a preset threshold intensity.
2. Description of the Related Art
It is often desirable to control the contrast of specific regions of an image in a variety of applications despite extreme variations in brightness of light emanating from neighboring such regions. For example, intensified CCD cameras used for night vision suffer from blooming and halo, meaning that distant, bright sources of light look larger and closer than they are, because the excess charge on the CCD "spills over" into adjacent areas. This results in distortion and/or masking of the portion of the image originating near the bright light source. Some uses for localized contrast control might include a welding face mask which attenuates only the flare and sparks, or laser safety goggles which attenuate only the laser beam. Currently, there is no means available to control the intensity of an image, in real time, on a pixel by pixel basis.
It is known in the art to achieve desired output intensity of light by various means. U.S. Pat. No. 5,231,338 by Stalls discloses a color display system using Spatial Light Modulators (SLMs) wherein light from three primary beams is directed to a set of color regulating SLMs. Each one of these SLMs has an array of pixels which can be selectively switched on to get a desired intensity of color. U.S. Pat. No. 5,223,857 by Lace et al discloses a pulsed imaging, pulse width modulation raster output scanner which obtains three exposure levels by using pulse width modulation and spatial filtering. U.S. Pat. No. 5,206,629 by DeMond et al discloses an SLM and memory for digitized video and display with several redundant rows of cells permitting differing intensity by illuminating selected cells in each column. However, none of these patents are capable of resolving the aforementioned problem of blooming in that they do not disclose a method or apparatus for limiting output intensity by varying attenuation based on input intensity.
It is also known in the art to vary the brightness of an output image. U.S. Pat. No. 5,151,718 by Nelson discloses an SLM array which can be operated by varying the brightness of individual pixels to compensate for optical system deficiencies. The adaptation does not take place in real time. U.S. Pat. No. 4,961,615 by Owechko et al discloses an associative memory system with an SLM. The SLM is used in a liquid crystal light valve comprising a liquid crystal, a photoconductor, and a grid. The grid is in a checkerboard configuration in order to confine blooming to within each aperture. No on-pixel processing occurs.
It is known in the art to correct various imperfections introduced by the device in the optical signal produced. U.S. Pat. No. 5,119,214 by Nishii et al discloses an SLM pixel array wherein the amplitude component of each cell is corrected for phase distortion. U.S. Pat. No. 5,096,279 by Hornbeck et al discloses a method of resetting the pixels of an SLM in order to solve the problem of deformable mirrors sticking to the landing electrodes.
It is also known in the art to use the intensity of the input image to produce pseudocolor. U.S. Pat. No. 5,019,898 by Chao et al discloses a method of pseudocolor encoding an image. It alters the polarization of a beam using a spatial light polarization module in accordance with the spatial distribution of intensities in the image. No on-pixel processing is done.
A need remains in the art for apparatus and methods to adaptively attenuate an optical signal on a pixel by pixel basis.